1. Field of the Invention
The present invention relates to a liquid container for supplying a liquid such as ink to a pen or printing head as a printing section or to a liquid using apparatus such as a printing apparatus, for example, in an efficient and stable manner, and to an ink jet printing apparatus utilizing the liquid container.
2. Description of the Related Art
Ink jet printing apparatuses that form an image on a printing medium by applying ink that is a liquid to the printing medium using a liquid consuming or using apparatus such as an ink jet printing head include apparatus that form an image by ejecting ink while moving a printing head relative to a printing medium and apparatus that form an image by ejecting ink while moving a printing medium relative to a fixed printing head conversely.
Methods of supplying ink to a printing head used in such an ink jet printing apparatus include a method referred to as on-carriage method in which an ink tank is integrally or separably mounted to a printing head that is carried by a carriage to be moved back and forth (main scanning) and in which ink is directly supplied from the ink tank to the printing head. There is another method referred to as tube supply method in which an ink tank is fixed in a region of a printing apparatus other than a carriage as a body separate from a printing head carried by the carriage and in which ink is supplied by coupling the ink tank and the printing head through a flexible tube. The method includes a configuration in which a second ink tank to serve as an intermediate tank (sub-tank) between an ink tank (main tank) and a printing head is mounted on the printing head or a carriage and in which ink is directly supplied from the second ink tank to the printing head.
According to those methods, an ink tank to supply ink to a printing head directly is provided with a mechanism for generating an adequate negative pressure in a range in which the negative pressure is in equilibrium with a pressure in the printing head to hold meniscuses formed at an ink ejecting section thereof to prevent the ink from leaking from the ink ejecting section satisfactorily and in which an ink ejecting operation of the printing head can be performed.
In a negative pressure generating mechanism of this type, a porous member such as a sponge that is impregnated with ink to be held thereby is contained in an ink tank, and an adequate negative pressure is generated by an ink holding capacity of the same.
In another mechanism, a bag-shaped member formed from a material such as rubber having an elastic force and generating a tension in the direction of increasing the volume thereof is charged with ink as it is, and the tension generated by the bag-shaped member exerts a negative pressure to the in therein.
In still another mechanism, a bag-shaped member is formed using a flexible film, and a spring for urging the film in the direction of increasing the volume of the bag-shaped member is bonded to the interior or exterior of the same to generate a negative pressure.
In any of the above mechanisms, however, the negative pressure tends to increase as the amount of ink in the ink tank decreases, and it becomes impossible to supply ink to a printing head stably when the level of the negative pressure exceeds a predetermined value. This results in a problem in that the ink tank becomes unusable before the ink is completely used up.
Several mechanisms have been proposed as follows to prevent magnitude of a negative pressure being too much greater than the predetermined level.
For example, Japanese Patent Application Laid-open No. 7-125240 (1995) and Japanese Patent Application Laid-open No. 7-125241 (1995) have disclosed mechanisms in which a hydrophobic film and a tubular vent port are provided in a tank, and a spherical body is disposed in the tube to introduce air into the tank when a negative pressure therein increases. That is, those publications have disclosed mechanisms which have a tubular vent port (boss) that establishes communication between the outside and inside of a container and in which spherical body having an outer diameter smaller than an inner diameter of the boss is attached to a plurality of projecting ribs provided on an inner wall of the boss to form a substantially annular orifice with the spherical body and the boss. The size of such an orifice is chosen such that a small amount of ink is kept in the orifice as a liquid seal because of the capillarity of ink. The orifice is configured such that a negative pressure in the container overcomes the capillarity of ink to disable the liquid seal when it nearly reaches the limit of an operating range of the printing head. At the time, the atmosphere enters into the container in the form of bubbles which results in causing a reduction of an inner pressure of over a certain level.
Japanese Patent Application Laid-open No. 6-183023 (1994) has disclosed a mechanism in which a plate-like member having a hole and a plate having a protrusion are provided in a face-to-face relationship in an ink bag constituted by a flexible sheet with a spring member disposed between the plates and in which the protrusion enters the hole when an internal negative pressure exceeds a predetermined value to separate the plate having the hole and the flexible sheet from each other, thereby introducing air in the tank. In this mechanism, the plate having the hole and the flexible sheet come into tight contact with each other after air is introduced, and leakage of ink is prevented by an ability for holding ink meniscuses or a liquid seal formed between those elements.
However, those methods require a plurality of parts in a region where air is introduced, and the structure of such a region has therefore become complicated.
When a pressure in a container T having a certain amount of air introduced therein becomes extremely high as a result of an ambient change (a reduction of the atmospheric pressure or a temperature rise) as shown in FIG. 1A, ink is pushed out from the container as shown in FIG. 1B, which can result in leakage of ink through an ink ejection port N or a vent hole A when the container is used in an ink jet printing head. When a liquid is contained in a bag-shaped member constituted by a flexible sheet, although expected is a certain degree of buffering effect that moderates an increase of a pressure therein by accommodating expansion of air which results in a pressure reduction, such an effect is limited.
Referring to Japanese Patent Application Laid-open No. 7-125241 (1995) or Japanese Patent Application Laid-open No.6-183023(1994) in this connection, a method is disclosed in which a channel in the form of a maze is provided at the bottom of a tank and in which an ink overflow resulting from an increase in the internal pressure is allowed to move to that section. Although this method is effective, the formation of the maze-like channel results in a more complicated structure, and a certain degree of ink evaporation is unavoidable because the other end of the maze-like channel is always in communication with the atmosphere.
Further, in those examples of the related art, there is provided an opening section for directly introducing the atmosphere into an ink tank. As a result, the quantity of gases in the ink tank becomes relatively great in a region in the ink tank where ink is nearly used up depending on the size and position of the opening section, which can result in incomplete holding of meniscuses at the ink ejection port or opening section when the negative pressure is eliminated as a result of introduction of the atmosphere and can therefore lead to leakage of ink or incomplete introduction of the atmosphere.
Particularly, in the case that the opening section (a vent) is configured to have a contact with the ink stored therein directly, there arises a necessity to control a dimension and a shape of the opening area of the opening section precisely in order to avoid an ink leakage from the opening section.
In addition, breakage of a liquid seal can occur because of various conditions such as a difference between air pressures inside and outside the container, a temperature rise of drop, a shock or drop that occurs when the ink tank is handled alone, and acceleration that occurs during main scanning according to the serial printing method, in particular. This results in a problem in that air can be introduced or ink can leak out conversely even when a pressure in the container has not reached a predetermined value. Further, such conditions can vary depending on the designs of the printing head and ink tank or physical properties of ink, and a problem arises also in that designing must be adequately carried out in accordance with the shape and dimensions of the opening section and the basic configuration of the negative pressure generating mechanism depending on each mode of use.
The technique disclosed in the Japanese Patent Application Laid-open No. 6-183023 (1994) employs a structure in which air is introduced through a microscopic gap between a thin plate-like member and a flexible sheet. This has resulted in another problem in that a negative pressure becomes unstable when air is introduced because the force for causing separation as described is changed by a capillary force that is generated when a liquid enters the gap.